Purification and characterization of a mucin-binding mycelial lectin from Aspergillus nidulans with potent mitogenic activity

Microbial lectins as a potential therapeutics for the prevention of certain human diseases

Lectins are protein or glycoprotein molecules with a specific ability to bind to carbohydrates. From viruses to mammals, they are found in various organisms and exhibit remarkable diverse structures and functions. They are significant contributors to defense mechanisms against microbial attacks in plants. They are also involved in functions such as controlling lymphocyte migration, regulating glycoprotein biosynthesis, cell-cell recognition, and embryonic development in animals. In addition, lectins serve as invaluable molecular tools in various biological and medical disciplines due to their reversible binding ability and enable the monitoring of cell membrane changes in physiological and pathological contexts. Microbial lectins, often referred to as adhesins, play an important role in microbial colonization, pathogenicity, and interactions among microorganisms. Viral lectins are located in the bilayered viral membrane, whereas bacterial lectins are found intracellularly and on the bacterial cell surface. Microfungal lectins are typically intracellular and have various functions in host-parasite interaction, and in fungal growth and morphogenesis. Although microbial lectin studies are less extensive than those of plants and animals, they provide insights into the infection mechanisms and potential interventions. Glycan specificity, essential functions in infectious diseases, and applications in the diagnosis and treatment of viral and bacterial infections are critical aspects of microbial lectin research. In this review, we will discuss the application and therapeutic potential of viral, bacterial and microfungal lectins.

Purification and characterisation of a xylose-specific mitogenic lectin from Fusarium sambucinum

A xylose-specific intracellular lectin, showing hemagglutination only with rabbit erythrocytes was purified from mycelium of Fusarium sambucinum which was designated as FSL. An array of anion exchange chromatography on Q-Sepharose and gel-exclusion chromatography on Sephadex G-100 resulted in 84.21% yield and 53.99-fold purification of lectin with specific activity of 169.53 titre/mg. Molecular weight of FSL determined by SDS-PAGE was 70.7 kDa, which was further confirmed by gel-exclusion chromatography. Native-PAGE analysis of FSL showed its monomeric nature. FSL was observed to be a glycoprotein containing 2.9% carbohydrate. Hapten inhibition profile of FSL displayed its strong affinity towards D-xylose (MIC 1.562 mM), L-fucose (MIC 6.25 mM), D-mannose (MIC 3.125 mM), fetuin (MIC 15.62 μg/mL), asialofetuin (MIC 125 μg/mL) and BSM (MIC 3.125 μg/mL). Affinity of FSL towards xylose is rare. FSL was found stable over a pH range 6.0-7.5 and upto 40 °C temperature. Hemagglutination activity of FSL remained unaffected by divalent ions. Lectin concentration of 5 μg/mL was found sufficient to stimulate proliferation of murine spleen cells and its concentration 75 μg/mL exhibited highest mitogenic potential. FSL exhibited maximum mitogenic stimulatory index of 14.35. The purification, characterisation and mitogenicity of F. sambucinum lectin has been reported first time.

Purification of a potent mitogenic homodimeric Penicillium griseoroseum lectin and its characterisation

Penicillium griseoroseum lectin was 80-fold purified by successive DEAE Sepharose anion exchange and Sephadex G-100 gel permeation chromatography. P. griseoroseum lectin exhibited haemagglutination activity towards protease-treated rabbit erythrocytes. It showed specificity towards various carbohydrates such as d-mannose, N-acetyl-d-glucosamine, mucins, and so forth. P. griseoroseum lectin was found as a glycoprotein with glycan content of 4.33%. Purified P. griseoroseum lectin is homodimeric having a molecular mass of 57 kDa with subunit molecular mass of 28.6 kDa. Haemagglutination activity of purified P. griseoroseum lectin was completely stable from 25°C to 35°C at a pH range of 6-7.5. Lectin activity was not influenced by divalent metal ions and denaturants. P. griseoroseum lectin manifested mitogenicity towards mice splenocytes and activity reached a peak at 75 μg/ml of lectin concentration. P. griseoroseum lectin in microgram concentrations stimulated proliferation of mice splenocytes. Thus, P. griseoroseum lectin exhibits potential mitogenicity, which can be exploited for further biomedical applications.

A fucose specific lectin from Aspergillus flavus induced interleukin-8 expression is mediated by mitogen activated protein kinase p38

Aspergillus flavus is an ubiquitous, opportunistic fungus responsible to cause invasive fungal allergic diseases, including bronchopulmonary invasive aspergillosis in persons with altered immune function. Lectins have been implicated as interaction mediators between the pathogenic fungi and human host. We isolated L-fucose specific lectin from A. flavus (FFL) and purified it to homogeneity with a combination of ion exchange and hydrophobic interaction chromatography methods. Its hemagglutination activity was significantly inhibited by 125 μM L-fucose as compared to other sugars and sugar derivatives. We, then used human cell line L-132, and U937 cell line to explore the possible cytotoxicity and proinflammatory effect of this fucose-specific lectin. The lectin induced the expression of proinflammatory cytokine interleukin-8 (IL-8) in a dose-dependent manner, and it was found to be associated with the p38 mitogen activated protein kinase (MAPK). The p38MAPK signalling pathway regulates the transcription factor activator protein-1 (AP-1) activity, which is the integration point of many signals that can differentially affect the expression and transcriptional activity of a cell. We observed activation of c-Jun, a critical component of the AP-1 complex, mediated by p38MAPK upon the FFL treatment in L-132 cells. Finally, inhibition of p38MAPK by a specific inhibitor attenuates the c-Jun, suggesting the p38MAPK involvement in the c-Jun activation, which in turn transcriptionally activates the induction of IL-8 in response to the lectin. Thus, this study showed a potential lectin-mediated mechanism to modulate the immune response during host-fungus interactions.

Lectin activity in mycelial extracts of Fusarium species

Lectins are non-immunogenic carbohydrate-recognizing proteins that bind to glycoproteins, glycolipids, or polysaccharides with high affinity and exhibit remarkable ability to agglutinate erythrocytes and other cells. In the present study, ten Fusarium species previously not explored for lectins were screened for the presence of lectin activity. Mycelial extracts of F. fujikuroi, F. beomiformii, F. begoniae, F. nisikadoi, F. anthophilum, F. incarnatum, and F. tabacinum manifested agglutination of rabbit erythrocytes. Neuraminidase treatment of rabbit erythrocytes increased lectin titers of F. nisikadoi and F. tabacinum extracts, whereas the protease treatment resulted in a significant decline in agglutination by most of the lectins. Results of hapten inhibition studies demonstrated unique carbohydrate specificity of Fusarium lectins toward O-acetyl sialic acids. Activity of the majority of Fusarium lectins exhibited binding affinity to d-ribose, l-fucose, d-glucose, l-arabinose, d-mannitol, d-galactosamine hydrochloride, d-galacturonic acid, N-acetyl-d-galactosamine, N-acetyl-neuraminic acid, 2-deoxy-d-ribose, fetuin, asialofetuin, and bovine submaxillary mucin. Melibiose and N-glycolyl neuraminic acid did not inhibit the activity of any of the Fusarium lectins. Mycelial extracts of F. begoniae, F. nisikadoi, F. anthophilum, and F. incarnatum interacted with most of the carbohydrates tested. F. fujikuroi and F. anthophilum extracts displayed strong interaction with starch. The expression of lectin activity as a function of culture age was investigated. Most species displayed lectin activity on the 7th day of cultivation, and it varied with progressing of culture age.

Purification, characterization, and mitogenic potential of a mucin-specific mycelial lectin from Aspergillus sparsus

Lectins are carbohydrate binding proteins or glycoproteins that bind reversibly to specific carbohydrates present on the apposing cells, which is responsible for their ability to agglutinate red blood cells, lymphocytes, fibroblasts, etc. Due to their carbohydrate specificity, lectins have been used for purification and characterization of glycoproteins like antibodies, cytokines, tumor-associated glycoproteins, hormones, inhibitors, growth factors, various enzymes, membrane proteins (receptors), or even toxins and viruses. In the present study, a mycelial lectin from Aspergillus sparsus was purified, characterized, and evaluated for its mitogenic potential. Lectin could be effectively purified 17.8-fold in a single-step using affinity chromatography on mucin-sepharose column. Lectin migrated as a single band in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) with an apparent molecular mass of 100.2 kDa. Lectin showed pH optima of 6.5-8.0, and optimum temperature was determined to be 20-30 °C. Lectin was stable within a pH range of 5.5-10.0 and showed fairly good thermostability. Lectin activity was unaffected in the presence of EDTA, while activity reduced upon interaction with denaturants. MTT assay revealed strong mitogenic potential of A. sparsus lectin at a concentration up to 100 μg/ml.

Purification and characterization of a mucin specific mycelial lectin from Aspergillus gorakhpurensis: application for mitogenic and antimicrobial activity

BACKGROUND

Lectins are carbohydrate binding proteins or glycoproteins that bind reversibly to specific carbohydrates present on the apposing cells, which are responsible for their ability to agglutinate red blood cells, lymphocytes, fibroblasts, etc. Interest in lectins has been intensified due to their carbohydrate specificity as they can be valuable reagents for the investigation of cell surface sugars, purification and characterization of glycoproteins. The present study reports the purification, characterization and evaluation of mitogenic and antimicrobial potential of a mycelial lectin from Aspergillus gorakhpurensis.

METHODS

Affinity chromatography on mucin-sepharose column was carried out for purification of Aspergillus gorakhpurensis lectin. The lectin was characterized for physico-chemical parameters. Mitogenic potential of the lectin was evaluated against splenocytes of Swiss albino mice by MTT assay. Antimicrobial activity of the purified lectin has also been evaluated by disc diffusion assay.

RESULTS

Single-step affinity purification resulted in 18.6-fold purification of the mycelial lectin. The molecular mass of the lectin was found to be 70 kDa and it was composed of two subunits of 34.8 kDa as determined by gel filtration chromatography, SDS-PAGE and MALDI-TOF analysis. pH optima of the lectin was found to be 6.5-9.5, while optimum temperature for lectin activity was 20-30 °C. Lectin was stable within a pH range of 7.0-10.5 and showed fair thermostability. EDTA did not affect lectin activity whereas it was found susceptible to the denaturants tested. MTT assay revealed strong mitogenic potential of A. gorakhpurensis lectin at a concentration upto 150 µg/mL. Antimicrobial activity assay showed its potent antibacterial activity against Bacillus cereus, Staphylococcous aureus and Escherichia coli and marginal antifungal activity against Saccharomyces cerevisiae.

CONCLUSION

This is the first report on the mitogenic and antimicrobial potential of Aspergillus gorakhpurensis lectin. The results will provide useful guidelines for further research in clinical applications of this lectin.

Immunomodulatory and therapeutic potential of a mycelial lectin from Aspergillus nidulans

Lectins bind to surface receptors on target cells, and activate a cascade of events, eventually leading to altered immune status of host. The immunomodulatory potential of purified lectin from Aspergillus nidulans was evaluated in Swiss albino mice treated intraperitoneally with seven different doses of purified lectin. Lectin prevented BSA-induced Arthus reaction and systemic anaphylaxis. The enhanced functional ability of macrophages was evident from respiratory burst activity and nitric oxide production in splenocyte cultures. Interferon-gamma and interleukin-6 levels were significantly up-regulated in treated groups. Maximum stimulatory effect was observed at the dose of 1.5 mg/kg body weight. Therapeutic potential of A. nidulans lectin was assessed against trinitrobenzene sulfonic acid-induced ulcerative colitis in male Wistar rats. Rats pre-treated with 80 mg/kg body weight of purified lectin intraperitoneally prior to colitis induction showed lesser disease severity and recovery within 7 days, while rats post-treated with the same dose showed recovery in 11 days. The results demonstrate immunomodulatory effects of A. nidulans lectin in Swiss albino mice, resulting in improved immune status of the animals and unfold its curative effect against ulcerative colitis in rat model. This is the first report on immunomodulatory and therapeutic potential of a lectin from microfungi.